Combustion engines burn a mixture of air and fuel, generating mechanical power and a flow of exhaust. Engine exhaust contains, among other things, unburnt fuel, particulate matter such as soot, and harmful gases such as nitrous oxide or carbon monoxide. Modern engines are required to meet stringent emissions standards, which require the engines to discharge reduced levels of nitrous oxide and soot into the atmosphere. To comply with these standards, some engines use exhaust gas recirculation (EGR) circuit to recirculate a portion of the exhaust produced by the engines back through combustion chambers of the engines, which is known to reduce undesirable emissions discharged to the atmosphere.
An exemplary turbocharged engine implementing exhaust gas recirculation is disclosed in a technical article titled “Engine Gas Recirculation: Internal engine technology for reducing nitrogen oxide emissions” by MTU Friedrichshafen GmbH that published in August of 2011 (“the technical article”). In particular, the technical article discloses an internal combustion engine having a plurality of cylinders arranged into two different banks. Both banks of cylinders are provided with compressed air from a three-turbocharger arrangement (i.e., a high-pressure turbocharger and parallel low-pressure turbochargers). One of the banks of cylinders discharges exhaust to the turbochargers, while the other bank is considered a donor bank and discharges exhaust for recirculation within the engine. A single EGR cooler is mounted to the top of the engine for cooling exhaust from the donor bank of cylinders before the exhaust is distributed to all cylinders for subsequent mixing with air and combustion. A first control valve is located upstream of the EGR cooler and used to control EGR flow rates, while a second control valve is located in a bypass. The second control valve is used to build backpressure that drives the EGR process and also used to selectively direct excess exhaust from the donor bank of cylinders to the turbochargers.
Although the exhaust system of the technical article may provide for reduced emissions in some applications, it may still be less than optimal. In particular, the first control valve may be unable to independently vary the exhaust flows recirculated to the two banks of cylinders, which could limit engine functionality in some applications. Further, the location of the first control valve upstream of the cooler may create an extreme environment for the control valve that can lead to excessive wear of the valve. In addition, using the second control valve too frequently to both generate back pressure that drives EGR and to relieve excess exhaust from the donor bank could overtax the second control valve and cause it to fail prematurely.
The engine system of the present disclosure solves one or more of the problems set forth above and/or other problems in the art.